1. Field of the Invention
The present invention relates to a method for grinding a non-circular workpiece such as a cam shaft with a numerically controlled grinding machine.
2. Description of the Prior Art
Generally, in a numerically controlled grinding machine for grinding a non-circular workpiece, the feed of the grinding wheel perpendicular to a spindle axis is controlled according to profile data and machining cycle data.
The profile data comprises the amount of movement of the grinding wheel per unit angle rotation of the spindle which defines the profile generating movement of the grinding wheel along the finished shape of the workpiece. On the other hand, the machining cycle data comprises a numerical control program to control a machining cycle which includes rapid feed, cut-in feed and retracting feed of the grinding wheel. Workpieces are ground by the profile generating movement according to the profile data and the cut-in feed according to the machining cycle data.
In the grinding machine of the aforementioned type, the time relation between the cut-in feed and the profile generation movement of the grinding wheel is very important for desirable grinding accuracy and grinding speed.
In a prior grinding method, rotation of the main spindle is stopped during cut-in feed. Namely, the profile generating movement and the cut-in feed of grinding wheel are executed alternately. For example, the profile generating movement is stopped at a original angle position where the grinding point of the workpiece moves to the center of the base portion of the cam. Then, the grinding wheel is fed to the workpiece by a predetermined cut-in feed amount. After completing cut-in feed, the profile generating movement is resumed. The above-mentioned process is successively repeated.
In such grinding method, there are problems which occur regarding grinding speed and grinding accuracy. Namely, since the main spindle is stopped for cut-in feed during each rotation of the main spindle, the grinding speed is decreased. Furthermore, since the amount of deflection of the workpiece changes slightly between the process of cut-in feed and the process of profile generating movement due to stopping of the main spindle, a step is produced at the center of the base portion of the cam. Accordingly, in the prior grinding method, it is necessary to execute spark-out grinding for a long period of time so as to eliminate the step of the base portion.
In another prior method, the profile generating movement and the cut-in feed are executed simultaneously. Namely, the cut-in feed occurs continuously while the profile generation movement is executed. In such method, the grinding point of the workpiece is moved along a path of spiral shape. Accordingly, a large amount of the non-ground portion, the thickness of which is decreased in proportion to increase of rotational angle, remains when the cut-in feed is completed. Thus, it is necessary to execute spark-out grinding while the main spindle rotates more than one revolution after finishing cut-in feed.
Since a long period of spark-out grinding is needed in above-mentioned grinding methods, it takes a long time to grind the workpiece precisely. In the case of grinding a plurality of cam shafts which have a plurality of cams, such becomes a serious problem.